The present invention relates generally to satellite communication systems, and more particularly to a satellite communication system for communicating signals from a satellite within a constellation of satellites to multiple terrestrial antennas, which satellites transmit different signals simultaneously at designated frequencies, such as C-, Ku-, S-, L- and Ka-Band frequencies, but which frequencies are often nearly identical. The present invention also relates generally to terrestrial antennas for receiving signals from satellites, and more particularly to a terrestrial antenna for receiving a signal being transmitted from a satellite within a constellation of satellites, which transmit television signals in designated frequency bands, such as C-, Ku-, S-, L- and Ka-Bands. Finally, the present invention relates to the components for use in the above mentioned satellite communication system, such as the receiver, the transmitters and the associated satellites.
Television has evolved from a local broadcasting concept to a system in which a viewer may receive television signals from a variety of sources. Today, television viewers receive programming from at least one of several different methods, such as direct "over-the-air" broadcasts from local television stations, transmission over land cables, i.e., cable television (CATV), transmission over microwave systems, and direct to home (DTH) broadcast via satellite.
Television viewers may receive DTH satellite broadcasts by purchasing home satellite dish equipment, however, current satellite television communication systems operate with receiving antennas that are relatively large, e.g., on the order of 10 feet in diameter or more for current C-Band parabolic dishes. At lower satellite frequencies the receiving dishes are even larger.
The consequences of large receiving antennas affect the very nature of the type of service provided via satellite. Large dishes require a concrete pad for support, large amounts of space, installation by trained technicians and complicated positioning mechanisms due to the weight of the antenna, all of which translate to high costs for the initial installation. The high installation costs directly impact sales as many consumers cannot afford these high installation costs.
While cost is a large factor, it is by no means the only disadvantage of current DTH satellite service. Many consumers dislike the aesthetics of a large satellite dish sitting in their yard. Consequently, many consumers who could otherwise afford to subscribe to current DTH service do not subscribe because they do not want to place a large parabolic dish antenna in their yard. Due to the poor aesthetics of these antennas, restrictive covenants in housing developments often prohibit home owners from erecting them.
The combination of high costs and low aesthetics of these antennas limits the appeal of DTH satellite broadcasts, which directly competes with current CATV providers, hence the growth of CATV has been comparatively explosive due to lack of effective competition. However, CATV will probably never be available to all consumers due to its high installation costs in rural areas. Furthermore, the high costs of CATV installation means that many third world countries will not get CATV for many years, if ever. Thus, there will probably always be a market for DTH satellite services.
Even if the costs and aesthetic problems were solved, large antennas are not practical. While large dish antennas may be suitable for use in some applications, they are much too large for general home consumer use, or at least for use in most homes. The problem is particularly acute in urban areas, where it would be impractical for everyone to employ such a large antenna due to space limitations. As a result, CATV enjoys a relative monopoly on television services in urban areas.
In some parts of the world, other DTH services called Direct Broadcast Services (DBS) are available. The major advantage of these systems is they transmit signals at Ku-Band frequencies, which are higher than C-Band frequencies. Higher transmitting frequency permits a smaller receiving antenna, which for Ku-Band systems is on the average about 3 feet in diameter.
While higher frequency signals permit smaller receiving antennas, even these antennas can be too large for some applications where space is at a premium. Thus, there is a need for reducing the size of television antennas, particularly at lower satellite frequencies, such as C-Band frequencies or lower.
In addition, the demand for television services from satellites has caused the Federal Communications Commission (FCC) to approve narrower spacings in the synchronous orbits, about 22,000 miles above the earth's equator. The use of .+-.2.degree. spacing allows many satellites to supply television service to the U.S. market. As more and more DTH services become available, demand will cause further reductions in satellite spacing, making the problem of interference from adjacent satellites more acute.
Previously, it was believed that C-Band satellites because of their power limitations and close spacing (about .+-.2.degree.) in a synchronous satellite orbit were limited to receiving antennas at least 8 feet in diameter, and in most areas, commonly 10 feet to 15 feet in diameter. These large antennas are commonly used today, and more than 4 million such antennas are installed throughout the United States. These antennas receive television programs from up to 18 satellites. If satellite spacing reduces further, larger receiving antennas will be required to discriminate between the desired satellite and its closest neighbors.
Prior to the present invention, the only way to reduce the size of the receiving antenna for DTH systems to something below three feet in diameter was to use a higher radio frequency band, such as Ku-band (about 17 GHz), which is allocated by the FCC for direct broadcast services (DBS). In this radio band, the FCC permits higher power satellite transmissions, which translates to a reduction in the required antenna size. The higher frequency also results in a smaller width of the antenna sensitivity beam, as a result of the relationship between the width of the beam and the radio frequency. For example, a two to three foot diameter antenna operating at Ku-band frequencies, using a beam width of approximately 1.3.degree. to 1.5.degree. , typically can achieve an antenna sensitivity pattern that is sufficient to isolate signals from satellites that are .+-.2.degree. from the targeted satellite.
The move to higher frequency, however, comes at the cost of a need for even higher transmission power due to rain absorption at these higher frequencies. Rain has two effects on radio waves passing through it. Rain scatters the energy so that less of the energy reaches the receiver, and rain radiates thermal energy that reaches the receiver, thus increasing the random noise that interferes with the received signals. The amount of absorption and increased thermal noise from scattering is more severe for radio signals at higher radio frequencies and therefore with shorter wavelengths. The overall effect of rain loss depends on the level of rain expected and the reliability required for the service. For typical reliability levels of DTH service, at Ku-Band frequencies, one must increase the radiated power by a factor of ten to allocate for rain loss. About one third of the increase is due to increased noise and two thirds is due to rain absorption. For lower level frequency bands, such as C-Band, the corresponding allocation of power increase for the same level of reliability amounts to only about 30%.
By increasing the satellite transmission frequency to Ku-Band, higher power can be transmitted from the satellite, and a smaller antenna will achieve the required isolation for a .+-.2.degree. satellite spacing. For example, a three foot antenna operating at Ku-Band has a beam width of approximately 1.8.degree.. However, Ku-Band also requires a tenfold increase (1,000%) in transmitted power to overcome losses due to rain. At C-Band an increase of only 30% is typically needed for rain loss. Thus, merely moving to a higher frequency does not necessarily solve all the problems with antenna size.
In addition, to implement a small receiving antenna using existing C-Band satellites would seem to violate basic limitations on power and beam isolation. The restriction on total satellite power is set by the FCC at -152 dBW/m.sup.2 per 4 kHz bandwidth power flux density reaching the ground. The FCC limits vary with frequency. A higher power is permitted at Ku-Band frequencies. In fact, no limits exist for frequencies in the Ka-Band. The FCC limits are designed to protect ground microwave relay equipment from interference by satellite transmissions. Obviously, foreign governments have their own limits on radiated power. The present C-Band satellites operate with radiated power up to approximately 36 dB EIRP, which falls just below the FCC limit when reaching the ground. The normal way to achieve a ground station antenna area reduction is to increase the satellite power by an equal amount. A reduction from a nine or ten foot satellite antenna to a three foot antenna would normally require a tenfold increase in satellite power, which would significantly exceed the FCC imposed limits by approximately a factor of ten.
Reducing the antenna size and increasing the transmission power, even if permitted by the FCC, would not completely solve the problem because a small receiving antenna has a larger directional receiving range. A smaller antenna of normal design will receive the signal from the satellite of interest, but will also receive interfering signals from other satellites in the constellation, at least as currently configured in the C-Band system, for example. The received signal will thus be so distorted as to impair proper decoding and reception.
Thus, the other barrier to antenna size reduction is a corresponding increase in the beam width of the receiving antenna. Current eight foot C-Band antennas have beam widths typically of 1.8.degree., which is sufficient to discriminate between satellites .+-.2.degree. away in orbit. A normal three foot antenna has a beam width of approximately 4.9.degree., which is not sufficient to discriminate against satellites at .+-.2.degree. from the targeted satellite.
The power and beam width limitations are the main barriers that have prevented the industry from offering television services to small antennas at C-Band, which has in turn limited the growth of the DTH industry. To offer DTH service to small C-Band antennas, both power and beam width problems must be solved simultaneously.
Thus, the present invention is directed to the problem of solving the power and beam width limitations necessary to reduce the size of the receiving antenna in a satellite communication system. The present invention is also directed to the problem of developing a satellite communication system that permits the use of a relatively small receiving antenna, yet operates within the current FCC power limitations and with existing satellite configurations, which system will operate in at least C-, Ku-, S-, L- and Ka-Bands. The present invention is also directed to the problem of developing a terrestrial antenna for use in the above communication system that is relatively small, yet permits reception from existing satellite communication systems, without requiring a change in the FCC satellite transmission power limitations or a change in orbital locations of the satellites. Finally, the present invention is directed to developing the components for use in the above mentioned communication systems.